Semiconductor devices such as logic and memory devices are typically fabricated by a sequence of processing steps applied to a specimen. The various features and multiple structural levels of the semiconductor devices are formed by these processing steps. For example, lithography among others is one semiconductor fabrication process that involves generating a pattern on a semiconductor wafer. Additional examples of semiconductor fabrication processes include, but are not limited to, chemical-mechanical polishing, etch, deposition, and ion implantation. Multiple semiconductor devices may be fabricated on a single semiconductor wafer and then separated into individual semiconductor devices.
Optical metrology processes are used at various steps during a semiconductor manufacturing process to detect defects on wafers to promote higher yield. Optical metrology techniques offer the potential for high throughput without the risk of sample destruction. A number of optical metrology based techniques including scatterometry and reflectometry implementations and associated analysis algorithms are commonly used to characterize critical dimensions, film thicknesses, composition and other parameters of nanoscale structures.
As devices (e.g., logic and memory devices) move toward smaller nanometer-scale dimensions, characterization becomes more difficult. Devices incorporating complex three-dimensional geometry and materials with diverse physical properties contribute to characterization difficulty.
Spectroscopic ellipsometry (SE) is commonly used to measure thin films, including the optical properties and thickness of film stacks. In general a SE system is based on the polarimetric principle. A light beam with a defined polarization state is directed to a sample. After interacting with the sample, the polarization state of the incident beam is modified. This modification of polarization state manifests itself as a change in the magnitude and phase of two orthogonally polarized components. For the case of linearly polarized light, the two orthogonally polarization components are commonly referred to as s- and p-polarization. In a semiconductor fabrication environment, ellipsometric measurements are typically performed on materials (e.g., silicon, silicon dioxide, silicon nitride, photoresist, etc.) that are optically isotropic. To obtain satisfactory measurement results it has been necessary to build SE systems with a relatively large angle of incidence (AOI) of the incident beam on the sample. For example, for a rotating compensator spectroscopic ellipsometer (RCSE) an AOI is typically set at 60 degrees and higher. In another example, for a rotating polarizer spectroscopic ellipsometer (RPSE) the AOI is typically set close to the Brewster's angle of silicon (i.e., approximately 70 degrees). For measurements of optically isotropic materials with typical RPSE and RCSE systems, selection of small angles of incidence results in substantially reduced measurement sensitivity.
As critical dimensions (e.g., the gate width or gate oxide thickness) continue to decrease, maintaining measurement precision (e.g., “3-sigma” measurement values) and tool-to-tool matching becomes progressively more difficult. As the values of the underlying structural parameters continue to shrink, the signal changes corresponding to a fractional change in the structural parameters become indistinguishable from system noise for typical RPSE or RCSE systems. As a result, both precision and tool-to-tool matching degrade.
Measurement precision and tool-to-tool matching are core challenges in the development of an optical metrology system that meets customer requirements of the semiconductor industry. Process and yield control in both the research and development and manufacturing environments demands increasing measurement precision and tool-to-tool consistency of measurement results. Thus, methods and systems for improved measurement sensitivity to meet the challenging CD and overlay metrology requirements are desired.